Superconducting metal-oxide compounds with a high transition temperature (T.sub.c) of, for instance, above 90 K, are generally known. These metal-oxide compounds are based particularly on a material system Me1-Me2- Cu- O, where the component Me1 comprises at least a rare earth metal including yttrium, and the component Me2 comprises at least an alkaline earth metal. Films or thin layers of these metal-oxide compounds are frequently produced by special vapor deposition or sputtering processes. In these processes a polycrystalline or amorphous preliminary product with the components of the chosen material system is first deposited on a suitable substrate. Generally, at this stage, the oxygen content is not adjusted exactly. This preliminary product is subsequently changed into the material with the desired super-conducting phase by means of a heat and oxygen treatment.
The metal-oxide phases which can be obtained in this manner, have a crystal structure similar to those of a perowskite and, in the case of YBa.sub.2 Cu.sub.3 O.sub.7-x, with 0&lt;x&lt;0.5, an orthorhombic structure (see, for instance, "Europhysics Letters" (Vol 3, No. 12, June 15, 1987, pages 1301 to 1307); the disclosure of which is hereby incorporated by reference). Since the materials having these superconducting phases must be counted among the oxide ceramics, the corresponding high-T.sub.c superconductors are frequently called oxide ceramic super-conductors.
Generally, single-crystal films of the system YBa.sub.2 Cu.sub.3 O.sub.7-x are produced by means of epitaxy on a monocrystalline SrTiO.sub.3 substrate. (See, for instance, "Physical Review Letters", Id.). On this substrate, which have an ordered crystal structure, and have a temperature of about 400.degree. C., the three metallic components are vapor-deposited from separate evaporation sources in an oxygen atmosphere, maintaining the desired stoichiometry. The preliminary product obtained in this manner, however, still has faults with respect to the desired superconducting high-T.sub.c phase. By means of a subsequent heat treatment, at a temperature of about 800.degree. to about 900.degree. C., while oxygen is being supplied, epitaxially grown monocrystalline, or at least strongly textured, film layers, with a desired superconducting high-T.sub.c phase, are obtained. Because of the strongly anisotropic crystal structure of this phase, the film layers so obtained also have anisotropic critical current densities (J.sub.c) The critical current density parallel to the surface of the respective layer is substantially higher than the critical current density in the direction of the normal of the layer and can be more than 10.sup.5 A/cm.sup.2 at 77 K. The aforementioned epitaxy is considered a necessary condition for reaching such high critical current densities. With this method, however, the cost of producing such layers is substantial. In addition, the epitaxy proceeding in the high-temperature process is difficult to control, since misoriented grains are frequently generated. These misoriented grains have an adverse effect on the superconducting properties, in particular the critical current density.
Besides the aforementioned vapor deposition technique, corresponding sputtering techniques, in which the sputtering on a substrate takes place from a target which contains at least the three metallic components of the material system, are also utilized (see, for instance, "Applied Physics Letters" (Vol. 51, No. 9, Aug. 31, 1987, pages 694 to 696; the disclosure of which is hereby incorporated by reference)). With the sputtering technique oxygen loading of the preliminary product at elevated temperature is also necessary.
Thus, in all generally known methods for producing thin layers of the high-T.sub.c superconducting material on specific substrates, higher critical current densities (above 10.sup.4 A/cm.sup.2), can be obtained only at considerable cost. These generally known methods are also difficult to reproduce.
The present invention, however, provides a solution to these problems, and allows high-T.sub.c superconducting layers with higher critical current densities (above 10.sup.4 A/cm.sup.2) to be produced, reproducibly, in a relatively simple manner.